1. Field of the Invention
The present invention generally relates to image forming apparatuses and, more particularly, to an image data correcting apparatus applicable to an image reading device or an image forming apparatus which can remove data corresponding to an image printed on a reverse side of a printed paper.
2. Description of the Related Art
When reading a both-side printed material such as newspaper, timetable, magazine, etc. by using a scanner or a copy machine, an image printed on a back side of the printed material may be undesirably read when reading an image on the front side of the printed material. Such a phenomenon is referred to as “back projection” or “reverse-side projection”.
Japanese Laid-Open Patent Application No. 3-068270 discloses an image processing apparatus that detects low intensity image data and calculates an average value to be set as threshold value so that image data having an intensity value below the threshold value is changed to image data corresponding to white.
Japanese Laid-Open Patent Application No. 8-340447 discloses an image forming apparatus which comprises back projection area determining means, back projection level detecting means, back projection level correcting means and back projection level correction value storing means so as to eliminate an image caused by a back projection.
Japanese Laid-Open Patent Application No. 9-116753 discloses an image reading apparatus which eliminates an image corresponding to a backing by previously reading a surface of the backing and subtracting image data corresponding to the backing from image data obtained by reading a document.
Japanese Laid-Open Patent Application No. 11-055524 discloses an image forming apparatus which calculates MTF of image data, and obtains an average value of the MTF so as to determine that the image data corresponds to a back projection when the average value is below a setting value, and applies a background process to the image data.
The methods disclosed in Japanese Laid-Open Patent Application No. 3-068270 and Japanese Laid-Open Patent Application No. 9-116753 may cause a problem of degradation in reproducibility (fading) of a low-intensity character image and an intensity jump in a highlight portion of a photographic image.
In the image forming apparatus disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 8-340447, since means for determining an area, in which back projection occurs, is provided, it is considered that the accuracy of detection of the area, in which the back projection occurs, must be high. In order to achieve such a high-accuracy detection, measures may be taken by performing pre-scanning or using character and picture separation algorithm. However, it is difficult to apply such a process to a relatively inexpensive apparatus and a high-speed machine, which requires real-time processing.
Besides, when performing elimination of back projection, there may be a case in which a low-intensity halftone image is eliminated since the low-intensity halftone image may be recognized as a back projection image.
A description will now be given, with reference to FIGS. 1A, 1B, 2A, 2B and 2C, of such a problem related to a low-intensity halftone image. FIG. 1A shows an example of image data obtained by reading a document of which both sides are printed. When viewing such a document from the front side, an image printed on the backside of the document may be visible due to transmission. In such a condition, the intensity of an image (back projection image) of the backside viewed from the front side is decreased, and an edge of the back projection image may be blunted, and data corresponding to such an image becomes that shown in FIG. 1B. The back projection image can be eliminated by changing data corresponding to the back projection image to a low intensity value corresponding to a background level (white). That is, if a difference in intensity within a small area of the document is below a setting value, the intensity distribution in the small area is regarded as flat, and, at this time, if the intensity of an image in the small area is below the setting value, the image is regarded as a back projection image.
However, in a case in which the image on the front side includes a low-intensity image such as a halftone image as shown in FIG. 2A, in particular, a color image, the halftone image may be recognized as a back projection image and is eliminated from image data to be reproduced as shown in FIG. 2C when an intensity difference of the halftone image is below a flat detection threshold value B as shown in FIG. 2B. This is because the intensity difference (calculated difference) between in a small area becomes that shown in FIG. 2B, and, therefore, the condition is established in which the image intensity is below the intensity threshold value E as shown in FIG. 2B and the intensity difference is below the flat detection threshold value B.